U.S. Bid to Prosecute BP Staff in Gulf Oil Spill Falls Flat.

NEW ORLEANS–A critical moment in the government’s case against Robert Kaluza, who was facing a criminal charge for his role in the 2010 BP PLC oil spill in the Gulf of Mexico, came when a former colleague was called to testify.

Donald Vidrine had already pleaded guilty and was expected to bolster prosecutors’ arguments. But he had trouble articulating exactly what he–and by extension Mr. Kaluza–may have done wrong. “I, we, uh, I may not have, I probably didn’t press hard enough,” the Louisiana native told a federal jury in New Orleans last week, after a long pause. “I thought I had.”

It was an ignominious end to the final case in the government’s effort to find individuals criminally responsible for the blowout on the Deepwater Horizon. In the four years since the U.S. began its cases against five men, prosecutors withdrew 23 counts before trial, judges dismissed 23 others and jurors acquitted on three counts. The three guilty pleas the government secured were all misdemeanors, and the men received or will likely receive probation.

The outcome, stemming from the largest oil spill off the U.S. coast, in which 11 people died and more than three million barrels of oil poured into the Gulf, is a reminder how hard it is to find individuals culpable for catastrophes where companies were held responsible.

Defendants and their attorneys successfully argued that many people made errors contributing to the disaster and the problems with the cleanup. Judges dismissed charges, implying that prosecutors had overreached, and one made clear he thought the charges had turned a series of mistakes into a criminal act.

When David Rainey, BP’s head of Gulf exploration and the most senior executive to face charges, was acquitted on a false-statements charge last year, the judge in the case, Kurt Engelhardt, said in court he thought it was the “correct verdict.”

BP’s penalties

In 2012, BP paid $4 billion in criminal penalties and pleaded guilty to an array of crimes connected to the spill, including manslaughter charges based on Messrs. Kaluza and Vidrine’s alleged negligence. The outcome of individual cases means BP is in the odd position of having pleaded guilty to crimes tied to charges against its employees that were dismissed by courts.

BP spokesman Geoff Morrell declined to comment for this article.

Early after the disaster, in 2011, the Justice Department consolidated all the BP investigations into a special task force, giving it a status reached by few cases besides the accounting fraud at now-defunct Enron Corp. Under pressure from Congress and the public after the 2008 financial crisis, in which few executives were charged with crimes, it brought around 50 criminal charges, some carrying penalties of up to 10 years in prison.

Prosecutors alleged that Mr. Rainey obstructed Congress and lied about how much oil was leaking from the well. A BP drilling engineer, Kurt Mix, was charged with obstructing justice by deleting text messages related to efforts to stop the spill. Anthony Badalamenti, a manager at Halliburton Energy Services Inc., which provided services to the drilling rig, was also charged with destroying evidence.

Messrs. Kaluza and Vidrine, the top supervisors at the well site, faced the gravest accusations, stemming directly from the death of 11 men. Each was charged with 11 counts of “seaman’s manslaughter”–an 1830s-era statute enacted to address steamboat collisions–for negligence leading to death at sea, plus 11 more of involuntary manslaughter based on “gross” negligence, each carrying a potential penalty of 10 years in prison.

“Make no mistake: While the company is guilty, individuals committed these crimes,” then-Justice Department official Lanny Breuer said in announcing the cases in November 2012.

A lawyer for Mr. Kaluza, David Gerger, said: “We are grateful for the jury: they understood technical evidence, and justice was done.” A lawyer for Mr. Vidrine, Bob Habens, declined to comment.

Two lawyers for Mr. Rainey, Reid Weingarten and Brian Heberlig, said “the Deepwater Horizon explosion was an accident, not a crime, no matter how hard prosecutors tried to make it one.”

A lawyer for Mr. Mix, Joan McPhee, said the prosecutors ignored evidence of Mr. Mix’s innocence and “sought to fill the gaping holes in its case by constructing a motive theory that had no grounding in fact.”

A lawyer for Mr. Badalamenti, who pleaded guilty and received probation, didn’t respond to a request for comment.

Justice Department spokesman Peter Carr said prosecutors from the criminal and environmental divisions and the U.S. Attorney’s office in New Orleans had spent years “committed to ensuring that the victims of this tragedy obtain justice and that those responsible for one of the worst environmental disasters in our country’s history are held accountable.”

Prosecutors were stymied by unexpected hurdles, including when one case was largely dismissed in a way they couldn’t appeal, and a conviction vacated due to problems with the jury.

Through their investigation, federal prosecutors said evidence showed Messrs. Vidrine and Kaluza made consequential mistakes.

Mr. Kaluza was the top BP man on board the rig on April 20, 2010, in the hours before the explosion. He and Mr. Vidrine, who took charge of the platform that night, oversaw a pressure test that produced troubling results, and prosecutors claimed they ignored the readings and accepted illogical theories to explain the anomaly.

Pressure test

Experts in the investigation said the decision to dismiss a reading that showed 1,400 pounds of pressure on the drill pipe was reckless. Few could understand the explanation Mr. Kaluza gave to justify his decision. He suggested the pressure might be caused by something other than oil flowing up the pipe.

An email sent by Mr. Kaluza to colleagues to explain the theory five days after the blowout was met with skepticism. One BP vice president’s response was: “???????????????????????????????????????????????????…followed by roughly another 500 question marks,” as one judge described it. The email was also described in court documents.

“They should have stopped the job and diagnosed the problem,” prosecutor Jennifer Saulino argued at the New Orleans federal courthouse last week. “Mr. Kaluza should have called Houston for help, but he didn’t.”

In the evening after the pressure test, the pipe running from the subsea oil well to the drilling rig buckled around the time a surge of natural gas from the well ignited, causing an explosion.

Mr. Kaluza sent emails that prosecutors said showed flippancy in discussing the disaster. “It was a very exciting night when we all had to get off the drilling rig in the middle of the ocean at night with the rig exploding!!! :),” he wrote a friend.

Mr. Kaluza had only been on the rig for a few days, as a replacement for another employee who was onshore to receive training.

Within a year of the indictment, the prosecutors’ case had started to unravel. Among other things, they had to explain their use of the antiquated “seaman’s manslaughter” law.

In December 2013, a federal judge in New Orleans ruled the law applied only to personnel who perform marine and navigation operations, not drilling functions.

“The Court recognizes the soundness in the government’s argument that oil rig blowouts pose similar risks and safety concerns as did the steamboat explosions of the 19th and 20th centuries,” Judge Stanwood Duval Jr. wrote. “Yet it is for Congress to include such types of disasters within the scope” of the law.

Worse for the government, judges questioned the basis for the 11 counts of involuntary manslaughter, making points even the defendants hadn’t raised, conveying their sense that prosecutors were overreaching.

“It’s still a criminal response to a human error,” Judge Higginbotham said.

Justice Department officials ultimately decided new evidence from related civil trials made most of the case untenable, according to people familiar with the discussions. Prosecutors moved to withdraw the manslaughter charges in December.

That left Messrs. Kaluza and Vidrine each facing a single misdemeanor count of violating the Clean Water Act by negligently discharging oil into the Gulf. Mr. Vidrine pleaded guilty to that count on the condition he receive probation. Mr. Kaluza chose to fight the charge, leading to an eight-day trial that ended Thursday night with a rapid not-guilty verdict.

Prosecutors’ other BP cases also fared poorly. A judge tossed out the obstruction-of-Congress count against Mr. Rainey, BP’s Gulf exploration head, when the House of Representatives declined to make certain staffers available.

The ruling left them pursuing what was essentially an add-on count–a single statement Mr. Rainey made during a six-hour FBI interview that they alleged was false.

Some of Mr. Rainey’s witnesses brought the jury to tears. One who flew in from Angola to testify on his behalf impressed the judge. “I’m not quite sure I could get somebody to travel halfway across New Orleans to say something nice about me…So the fact that someone flew halfway across the globe to come and testify…I guess is a credit to you, sir,” Judge Engelhardt said in court.